Compliant material

ABSTRACT

A device including a first member joined to a second member, the joining being carried out via a compliant material having an alloy with at least two materials selected from silver, gold, and palladium, the second member being a shrink ring of the first member.

TECHNICAL FIELD

The invention relates in particular to an item of equipment chosen froma furnace, a boiler, a superheater, a steam generator, a chemicalreactor and a heat exchanger, intended in particular for the productionof ethylene. In particular, it relates to means for providing orimproving an end-to-end connection between a tube made of ceramic (inparticular of silicon carbide) and a metal tube, with or without directcontact between the two tubes.

STATE OF THE ART

The production of ethylene results conventionally from a steam crackingoperation starting from ethane or naphtha. To this end, an ethyleneproduction unit 5, as represented diagrammatically in FIG. 1,conventionally comprises a furnace 10 comprising a chamber 11 throughwhich passes a bundle of tubes 12 and which is heated by at least oneburner 13.

The tubes 12, of circular cross section, exhibit an external diameter ofbetween 50 and 250 mm and a wall with a thickness of between 5 and 25mm. These tubes, bent into a pin, exhibit a length inside the chamber ofthe furnace of several meters. They are made of an alloy of iron, nickeland chromium.

In order to produce ethylene, feed means 14 feed an upstream end of thetubes 12 with a reactive mixture M of hydrocarbons and of steam, atemperature of greater than 1000° C. being maintained in the chamber 11.The flow rate of this mixture is adjusted so that the reactive mixture Mcan reach a temperature of greater than 750° C. and can react to resultin the production of ethylene.

The production of ethylene results in a deposition of coke on theinternal surfaces of the tubes 12, bringing about a fall inproductivity.

In particular, it has been discovered that the nickel of the alloyforming the tubes catalyzes the production of coke. Researchers havethus envisaged replacing the metal tubes by nickel-free tubes, inparticular ceramic tubes. The ceramic tubes have to be able to beconnected to the remainder of the plant and in particular to metal tubesused to introduce the reactive mixture or to discharge the reactionproducts. Research studies have thus been carried out in order to findsuitable connecting devices, in particular devices capable ofwithstanding the thermal cycles, typically with an amplitude of morethan 700° C., which result from the shutdowns of the furnaces.

In particular, the ceramic tubes and the metal tubes exhibit verydifferent thermal expansion coefficients, typically of approximately4.10⁻⁶ K⁻¹ and of approximately 18.10⁻⁶ K⁻¹ respectively. In theapplications targeted, the connections tested result in losses inleaktightness, indeed even in separation of the tubes.

For example, FR 2 645 941 describes a connecting device between a metalsupport and a ceramic tubular element in which a collar is attached tothe support and surrounds the ceramic tube with a functional clearance.The leaktightness is obtained by a coke seal resulting from theoperation of the production unit. However, this functional clearanceresults, at the startup of the production unit, in escapes ofhydrocarbons.

In addition, the connection is not mechanically reliable, in particularat ambient temperature. It is thus not suitable for an environment suchas that of an ethylene production furnace.

Furthermore, decoking cannot be carried out with steam but necessarilyhas to result from mechanical abrasion.

Finally, the connecting device described in FR 2 645 941 can only beapplied in plants which generate coke.

SUMMARY OF THE INVENTION

There thus exists a need for a novel connecting device, suitable inparticular for the connection of a ceramic tube with a metal tube in anenvironment such as that of an ethylene production furnace, but withoutbeing limited thereto.

An object of the invention is thus to respond, at least partially, tothis need.

FIRST ASPECT OF THE INVENTION

According to a first aspect of the invention, this aim is achieved bymeans of a device comprising a first part shrink fitted by means of afirst shrink ring, the first shrink ring being itself shrink fitted bymeans of a second shrink ring exhibiting a lower thermal expansioncoefficient than the thermal expansion coefficient of the first shrinkring, at least between 20° C. and 1000° C.

At ambient temperature, the shrink rings provide a reliable mechanicalconnection and good leaktightness. During a rise in the temperature, thesecond shrink ring expands less than the first shrink ring. It thusexerts, on the latter, a compressive force which prevents it frombecoming detached from the first part or a loss in leaktightness betweenthe first part and the first shrink ring.

As will be seen in more detail in the continuation of the description,when the first part is a ceramic tube, it thus becomes possible, byattaching a metal tube to the first shrink ring, to obtain a connectionbetween the ceramic tube and the metal tube which is particularlyreliable mechanically. This connection can advantageously besufficiently leaktight, in particular for the targeted applications, notonly at ambient temperature but also over a temperature range of severalhundred degrees, typically of more than 700° C., of more than 800° C.,indeed even of more than 900° C.

A device according to the invention can also comprise one or more of thefollowing optional characteristics:

-   -   The first part is made of a material exhibiting a thermal        expansion coefficient of less than 10.10⁻⁶ K⁻¹, of less than        8.10⁻⁶ K⁻¹, of less than 6.10⁻⁶ K⁻¹ or indeed even of less than        5.10⁻⁶ K⁻¹, between 20° C. and 1000° C.    -   The first part is made of a material exhibiting a modulus of        elasticity (MOE) of greater than 150 GPa.    -   The first part is made of a ceramic, carbide, nitride or oxide        material capable of withstanding temperatures reaching 1400° C.,        indeed even 1500° C.    -   The first part comprises a ceramic or vitroceramic material.        This material can in particular be chosen from silicon carbide,        alumina, mullite, silicon nitride, zirconia, cordierite,        aluminum titanate and their mixtures.    -   The amount by weight of silicon carbide in the material of the        first part is greater than 80%, preferably greater than 90%.        Preferably, said material is composed of silicon carbide.        Silicon carbide exhibits properties which are of particular use        in the application targeted. In particular, it makes it possible        to limit the formation of coke and exhibits a high thermal        conductivity, allowing efficient transfer of the heat from the        furnace to the reactive mixture moving through the tubes. The        energy efficiency is thus better than that obtained with a        corresponding metal tube. Finally, silicon carbide, the maximum        operating temperature of which is greater than 2000° C., retains        its mechanical properties, even at very high temperature.        Advantageously, it thus becomes possible to increase the        temperature inside the furnace and, simultaneously, to increase        the flow rate of the mixture of reactants moving through the        tubes. The productivity of the furnace can thus be considerably        increased.    -   The material of the first part exhibits a total porosity of less        than 5%, preferably of less than 2%, indeed even of less than        1%. Advantageously, the permeability of the first part is        thereby reduced.    -   The first shrink ring comprises a metal. Preferably, the first        shrink ring is composed of one or more metals. In one        embodiment, it comprises a ferrous metal.    -   The first shrink ring is made of a material exhibiting a thermal        expansion coefficient of less than 25.10⁻⁶ K⁻¹ or of less than        20.10⁻⁶ K⁻¹ and/or of greater than 10.10⁻⁶ K⁻¹ or of greater        than 15.10⁻⁶ K⁻¹, between 20° C. and 1000° C.    -   The first shrink ring is made of a material exhibiting a melting        point of greater than 1200° C. Advantageously, the first shrink        ring can thus be heated to a very high temperature in order to        be fitted to the first part and/or during its use.    -   The material of the first shrink ring is chosen from        cobalt-based alloys, such as Stellite, austenitic steels,        ferritic steels or titanium-based alloys. In one embodiment, the        first shrink ring does not comprise copper and/or does not        comprise tin.    -   The first shrink ring is composed of just one material.    -   The first shrink ring exhibits the shape of a collar or of a        sleeve, the radial thermal expansion of which is not impeded. In        particular, the first shrink ring is not inserted into a part        which would impede its radial expansion outward. The fitting of        the first shrink ring is thereby simplified.    -   The ratio of the thermal expansion coefficient of the material        of the first shrink ring to the thermal expansion coefficient of        the material of the first part, which are measured at a        temperature between 20° C. and 1000° C., is greater than 2.5,        greater than 3 or greater than 4 and/or less than 6 or indeed        even less than 5.    -   The second shrink ring is made of a material exhibiting a        thermal expansion coefficient of less than 15.10⁻⁶ K⁻¹, indeed        even of less than 10.10⁻⁶ K⁻¹, and/or of greater than 4.10⁻⁶        K⁻¹, indeed even of greater than 6.10⁻⁶ K⁻¹, between 20° C. and        1000° C. The material of the second shrink ring can in        particular exhibit a thermal expansion coefficient of 8.10⁻⁶        K⁻¹, between 20° C. and 1000° C.    -   The second shrink ring is made of a material exhibiting a        melting point of greater than 1200° C. Advantageously, the        second shrink ring can thus be heated to a very high temperature        in order to be fitted to the first shrink ring, which makes it        possible to obtain a particularly tight connection.    -   The ratio of the thermal expansion coefficient of the material        of the first shrink ring to the thermal expansion coefficient of        the material of the second shrink ring, between 20° C. and 1000°        C., is greater than 1.2, greater than 1.5 or greater than 1.8        and/or less than 3.5, less than 3 or indeed even less than 2.5.    -   The second shrink ring is made of a material exhibiting a        modulus of elasticity (Young's modulus) at 20° C. of less than        110 GPa, preferably of less than 100 GPa. The material of the        second shrink ring can in particular exhibit a modulus of        elasticity (Young's modulus) of 90 GPa at 20° C.    -   The material of the second shrink ring exhibits a content by        weight of niobium and/or of Dilver P1 of greater than 80%,        preferably of greater than 85%, and/or of less than 95%,        preferably of less than 93%. Advantageously, the presence of        niobium makes it possible to obtain a dilatometric behavior        which is particularly well suited when the device is used for        the production of ethylene. Dilver P1 can advantageously be used        when the device according to the invention is not subjected in        service to temperatures of greater than 600° C. Under these        temperature conditions, niobium is preferably completely        replaced by Dilver P1. The modification to the content of        niobium and/or of Dilver P1 also makes it possible to precisely        adjust the thermal expansion coefficient to the targeted        application.    -   The material of the second shrink ring exhibits a content by        weight of hafnium, in the metal form, of greater than 5%,        preferably of greater than 8%, and/or of less than 15%,        preferably of less than 12%. Advantageously, the presence of        hafnium improves the high-temperature mechanical strength.    -   The material of the second shrink ring exhibits a content by        weight of titanium, in the metal form, of greater than 0.5%,        preferably of greater than 0.8%, and/or of less than 1.5%,        preferably of less than 1.2%. Advantageously, the presence of        titanium improves the ductility and the resistance to oxidation.    -   The material of the second shrink ring is composed of niobium        and/or of Dilver P1, of hafnium and of titanium, the remainder        to 100% being impurities representing less than 2%, preferably        less than 1%, as percentages by weight.    -   The second shrink ring is composed of just one material.    -   The second shrink ring exhibits the shape of a collar or of a        sleeve, the radial thermal expansion of which is not impeded. In        particular, the second shrink ring is not inserted into a part        which would impede its radial expansion outward. The fitting of        the second shrink ring is thereby simplified.    -   The first shrink ring and/or the second shrink ring extends over        a length of greater than 40 mm and/or of less than 150 mm.    -   The first shrink ring exhibits a thickness, preferably a        substantially constant thickness, of greater than 1 mm,        preferably of greater than 3 mm, more preferably of greater than        3.5 mm, and/or of less than 5 mm and/or the second shrink ring        exhibits a thickness, preferably a substantially constant        thickness, of greater than 3 mm, preferably of greater than 5        mm, preferably of greater than 8 mm, and/or of less than 15 mm,        preferably of less than 13 mm. These lengths and thicknesses        have proved to be optimal for ensuring good leaktightness of the        connection.    -   A second part is attached to the first shrink ring.    -   The second part is made of a material exhibiting a thermal        expansion coefficient of less than 25.10⁻⁶ K⁻¹ or of less than        20.10⁻⁶ K⁻¹ and/or of greater than 10.10⁻⁶ K⁻¹ or of greater        than 15.10⁻⁶ K⁻¹, between 20° C. and 1000° C. A device according        to the invention is in particular perfectly suitable for        connecting the first and second parts exhibiting very different        thermal expansion coefficients.    -   The material of the second part is metallic. In particular, this        material can be chosen from cobalt-based alloys, such as        Stellite, austenitic steels, ferritic steels, indeed even        ferrous steels, or titanium-based alloys.    -   The second part is made of a material exhibiting a thermal        expansion coefficient and/or a chemical composition which is/are        substantially identical to that/those of the first shrink ring.    -   The second part is welded or brazed to the first shrink ring.        The use of a weld or of a braze advantageously makes possible        reliable attachment, even at high temperature. It is        particularly well suited when the materials of the second part        and of the first shrink ring are similar, indeed even identical,        in composition.    -   The openings of the first shrink ring and/or of the second        shrink ring is/are cylindrical with a circular cross section. In        other words, the part exhibits, externally, a circular        cylindrical portion to which the first shrink ring is fitted        and/or the first shrink ring exhibits, externally, a circular        cylindrical portion to which the second shrink ring is fitted.        The first and second shrink rings can in particular exhibit the        general shape of collars.    -   The inside of the first part and/or of the second part is solid,        hollow or partially solid. The hollow internal volume can be of        any shape.    -   The first part and/or, if appropriate, the second part are        chosen from a tube and a solid bar.    -   The tube and/or the bar exhibit(s) a length of greater than 1 m,        preferably of greater than 3 m.    -   The greatest transverse external dimension of said tube or of        said bar (that is to say, the external diameter in the case of a        circular cross section), upstream and/or downstream of the        connecting device, is greater than 10 mm, preferably greater        than 20 mm, greater than 30 mm or greater than 40 mm and/or less        than 150 mm, less than 100 mm or less than 75 mm.    -   In a transverse cross section of said tube, upstream or        downstream and/or in the connecting device, the outline of the        internal surface is identical in shape to or different in shape        from the outline of the external surface of said tube. For        example, the internal surface may exhibit grooves or bumps or be        polygonal, for example square, and the external surface may be        circular.    -   In a transverse cross section of the first shrink ring, the        outline of the internal surface of the first shrink ring is        identical in shape to or different in shape from the outline of        its external surface.    -   A third part is attached to said second part.    -   The device comprises neither screws nor bolts. This is because        these parts are liable to break under the effect of an        encrustation of coke in their threads.    -   The first part is a ceramic tube or a solid ceramic bar and/or,        if appropriate, the second part is a metal tube or a solid metal        bar.    -   In one embodiment, the first and second parts are first and        second tubes axially aligned. Preferably, they are fitted end to        end, preferably without contact with one another.    -   In one embodiment, the first shrink ring projects from the first        part, that is to say, in the case where the first part is a tube        or a bar, extends beyond the axial end of the first part to        which it is fitted, and the second shrink ring does not project        from the first part, indeed even is fitted flush to the first        part.    -   In one embodiment, said first and second tubes exhibit the same        internal diameter.    -   Preferably, said first and second tubes are axially distant from        one another by less than 5 mm, less than 3 mm or less than 1 mm.    -   The first and/or second parts are tubes which preferably extend        over a length (along the axis of the tube) of greater than 10        cm, of greater than 50 cm, indeed even of greater than 1 m,        beyond the axial end of the first shrink ring extending in said        transverse plane.    -   The first part is a ceramic tube, preferably extending along a        substantially vertical axis, and the second part is a metal        tube. The ceramic tube can in particular exhibit a length of        greater than 1 meter, of greater than 5 meters, indeed even of        greater than 8 meters. The metal tube can in particular exhibit        a rectilinear axis and a position coaxially in the extension of        the ceramic tube. In an alternative form, the metal tube can be        angled, indeed even be U-shaped. In one embodiment, the two        branches of the U are connected to ceramic tubes, on each        occasion by means of a connecting device according to the        invention.

The first and second parts can have any conformation. However, at theconnecting device, the dimensions of the second part must allow it to beattached to the first shrink ring, itself attached externally to thefirst part.

In one embodiment in which the first and second parts are tubesexhibiting, over their entire length, including at the connectingdevice, an external diameter and a wall thickness which aresubstantially constant, this requirement thus results in a reduction inthe passage cross section at the transition between the second part andthe first part.

This is why, preferably, the first and second parts are tubes (or bars)exhibiting, over their entire length, a constant external diameterexcept, as regards the second part, at the connecting device. In thisembodiment, such as that represented in FIG. 3, the second part is thusa tube exhibiting a broadened axial end, for example in the form of acylindrical skirt, the dimensions of which are adjusted in order for itto be able to be used for the attaching to the first shrink ring.

The broadened axial end can be integral with the remainder of the tube.The practical preparation of a second part exhibiting such aconformation may, however, be problematic. In one embodiment, saidbroadened axial end is thus attached to a conventional tube (with aconstant diameter over its entire length) in the form of a junctionpart. For example, a skirt exhibiting an internal diameter greater thanthe internal diameter of such a tube can be welded to one end of saidtube.

Advantageously, the passage cross section can thus be kept constant,whatever the axial position under consideration (upstream, downstream orin the connecting device).

The use of a junction part is particularly effective when the secondpart is a solid bar.

The above characteristics and those of the other aspects of theinvention which are described below are advantageously incorporatedtogether in one and the same device in order to obtain an optimalconnection, although, in some cases, the advantages which result fromthese characteristics may be taken advantage of separately from theothers, in different devices.

The invention also relates to a process for joining a first part made ofa material exhibiting a first thermal expansion coefficient and a secondpart made of a material exhibiting a second thermal expansioncoefficient greater than said first thermal expansion coefficient, saidprocess comprising the following stages:

-   1) shrink fitting a first shrink ring to the first part;-   2) shrink fitting a second shrink ring to the first shrink ring,    said second shrink ring being made of a material exhibiting a    thermal expansion coefficient which is lower than the thermal    expansion coefficient of the first shrink ring;-   3) independently of stage 1) or stage 2), preferably after stage 2),    attaching the second part to the first shrink ring.

Preferably, the first part, the second part, the first shrink ring andthe second shrink ring are chosen so as to obtain, on conclusion ofstage 3), a device in accordance with the invention, according to anyone of its aspects.

The invention also further relates to a process for adapting a shrinkring to an environment exhibiting a specific variability in temperature,for example with an amplitude of greater than 200° C., of greater than400° C., of greater than 700° C., indeed even of greater than 900° C.,said process consisting in modifying the composition of said shrinkring, in particular in modifying the content of niobium and/or of DilverP1.

The shrink ring can in particular exhibit one or more characteristics,possibly optional, of a second shrink ring according to any one of theaspects of the invention. The material of the shrink ring can inparticular exhibit a content by weight of niobium and/or of Dilver P1 ofgreater than 80%, preferably of greater than 85%, and/or of less than95%, preferably of less than 93%.

SECOND ASPECT OF THE INVENTION

The fitting of a first shrink ring to a first part consistsconventionally in axially aligning the first shrink ring, heated to hightemperature, and a corresponding cylindrical portion of the first partand in then axially displacing the first shrink ring with respect to thefirst part in order to shrink fit the first shrink ring to the firstpart.

The axial alignment of the first part and of the first shrink ring hasto be sufficiently precise and the first part and the first shrink ringhave to be given dimensions with the smallest possible tolerance.

The physical proximity of the first shrink ring and of the first partduring the fitting can result in the appearance of thermal shocks. Inparticular, the low clearance can result in rapid heat transfers, inparticular in the case of contact between the first shrink ring and thefirst part, and especially when the first part exhibits a high thermalconductivity, for example because it is made of silicon carbide.

These heat transfers can damage the first part. Furthermore, they canresult in an expansion of the first part and in a contraction of thefirst shrink ring resulting in the first shrink ring being locked inposition before it has been able to reach the desired definitiveposition.

There thus exists a need for a shrink fitting process which makes itpossible to at least partially solve this problem.

One aim of the invention is to meet, at least partially, this need.

According to a second aspect of the invention, this aim is achieved bycoating the first part and/or the first shrink ring with a coating madeof a material exhibiting a lower thermal conductivity than that of thematerial of the first part.

This coating thus limits the heat exchanges between the first part andthe first shrink ring and thus reduces the risk of thermal shock and oflocking in an intermediate position, in particular in the event ofcontact during the fitting.

Preferably, the coating material exhibits a thermal conductivity of lessthan 10 W.m.° C. and, preferably, the ratio of the thermal conductivityof the material of the first part to the thermal conductivity of thecoating material is greater than 10.

The coating material is preferably a refractory material. In particular,the coating material can be chosen from zirconia, doped zirconia,cordierite, alumina, mullite, aluminum titanate, yttrium oxide,magnesium oxide, hafnium oxide or a mixture of these materials. Acoating comprising more than 80% by weight of zirconia has proved to beparticularly effective.

The coating material might also be made of metal.

The first part and/or the first shrink ring may or may not be a firstpart and/or a first shrink ring of a device according to one or moreother aspects of the invention.

THIRD, FOURTH AND FIFTH ASPECTS OF THE INVENTION

The inventors have found that, when a first shrink ring is fitted to thefirst part, material can migrate from the first shrink ring toward thefirst part. In particular, they have found that nickel from the firstshrink ring can migrate toward the silicon carbide of the first part,resulting in regions of stresses in the first part, which can result inbreakages during temperature changes.

There thus exists a need for a device comprising a first shrink ringfitted to a first part and exhibiting an improved mechanical strength.

Furthermore, when the first part is made of a material based on asilicon compound (that is to say, comprises more than 50% by weight ofsaid silicon compound), in particular when the first part is made ofsilicon carbide, it may be difficult to make a coating made of a ceramicoxide adhere to the surface of the first part.

This adhesion is particularly problematic when the material of the firstpart exhibits a total porosity of less than 5%.

There thus exists a need for a process for coating a substrate based ona silicon compound by means of a coating comprising at least one ceramicoxide, this process making it possible to attach said coating in areliable and lasting fashion.

An objective of the invention is to provide a technical solution forsolving, at least partially, the abovementioned problems.

According to a third aspect of the invention, this aim is achieved bymeans of a process for coating a substrate based on a silicon compoundby means of a coating comprising at least one ceramic oxide, saidprocess comprising the following successive stages:

-   a1) application to said substrate, preferably at ambient    temperature, of particles comprising, preferably composed of, said    at least one ceramic oxide;-   b1) heat treatment of said substrate under conditions suitable for    causing said at least one ceramic oxide to react with silicon of the    substrate so as to form a coating made of said at least one ceramic    oxide and a transition layer comprising silica at the transition    between said substrate and said coating;-   c1) optionally, application of a layer of a compliant material at    the external surface of said coating, said compliant material being    in accordance with the eleventh aspect of the invention described    below;-   d1) optionally, fitting a first shrink ring so that said first    shrink ring covers, at least partially, preferably completely, said    coating.

Alternatively, according to a fourth aspect of the invention, this aimis achieved by means of a process for coating a substrate based on asilicon compound by means of a coating comprising at least one ceramicoxide, said process comprising the following successive stages:

-   a2) oxidation, at least at the surface df said substrate, of said    silicon compound, for example by a heat treatment under an oxidizing    atmosphere, so as to form silica on said surface,-   b2) application to said substrate, preferably at ambient    temperature, of particles comprising, preferably composed of, said    at least one ceramic oxide;-   c2) heat treatment of said substrate under conditions suitable for    causing said at least one ceramic oxide to react with said silica so    as to form a coating made of said at least one ceramic oxide and a    transition layer comprising silica at the transition between said    substrate and said coating;-   d2) optionally, application of a layer of a compliant material at    the external surface of said coating, said compliant material being    in accordance with the eleventh aspect of the invention described    below;-   e2) optionally, fitting a first shrink ring so that said first    shrink ring covers, at least partially, preferably completely, said    coating.

Alternatively again, according to a fifth aspect of the invention, thisaim is achieved by means of a process for coating a substrate, inparticular based on a silicon compound, for example based on SiC, onsilicon nitride Si₃N₄ or on SiAlON, by means of a coating comprising atleast one ceramic oxide, said process comprising the followingsuccessive stages:

-   a3) deposition of a layer of silicon at the surface of the    substrate, preferably by plasma spraying;-   b3) application to said silicon layer, preferably at ambient    temperature, of particles comprising, preferably composed of, said    at least one ceramic oxide;-   c3) optionally, heat treatment of said substrate in order to cause    said at least one ceramic oxide to react with silicon of the    substrate so as to form a coating made of said at least one ceramic    oxide and a transition layer comprising silica at the transition    between said substrate and said coating;-   d3) optionally, application of a layer of a compliant material at    the external surface of said coating, said compliant material being    in accordance with the eleventh aspect of the invention described    below;-   e3) optionally, fitting a first shrink ring so that said first    shrink ring covers, at least partially, preferably completely, said    coating.

Stage c3) can be carried out like stage b1). However, in this process,advantageously, no heat treatment is necessary if, in stage b3), theapplication of the particles is carried out by thermal spraying. Theprocess is advantageously simplified thereby.

Whatever the process according to the invention for coating a substratebased on a silicon compound, the natures of the coating and of thesilicon compound of the substrate are not limiting. In particular, thecoating and/or the substrate can comprise one or more characteristics ofthe coating and of the substrate which are defined in the sixth aspectof the invention.

The above processes may also exhibit one or more of the followingoptional characteristics:

-   -   The silicon compound is a nonoxide compound. This is because the        processes are particularly well suited to silicon compounds of        this type.    -   In stage a1) or b2), the particles of said at least one ceramic        oxide are applied by spraying, painting or dipping. To this end,        a slip is prepared and is applied to the substrate. Preferably,        the slip does not comprise silica and preferably does not        comprise a silicon compound.    -   In stage b1) or c2) or c3), the substrate is subjected to a        firing at a temperature of between 1000° C. and 1400° C.,        preferably for a period of time of greater than 1 hour.    -   Stage b1) or c2) or c3) is carried out in situ, that is to say        after the coated substrate has been installed in its operating        position. Advantageously, the process is simplified thereby and        is reduced in cost thereby. For example, when the substrate is a        first part of a connecting device according to the invention,        this stage can be carried out after definitive attachment of        said device, for example after interposition between two tubes        positioned within an ethylene production furnace or within a        heat exchanger. The firing thus results from the first use of        the connecting device.

The invention also relates to a part comprising a coated substrateaccording to a process according to any one of the third, fourth andfifth aspects of the invention.

SIXTH ASPECT OF THE INVENTION

According to a sixth aspect, the invention also relates to a “coatedpart” comprising a substrate made of a material based on a siliconcompound and a coating made of a coating material comprising at leastone ceramic oxide, a transition layer comprising silica extendingbetween the substrate and the coating.

This coated part may exhibit one or more of the following optionalcharacteristics:

-   -   The transition layer exhibits a thickness of greater than 0.1 μm        and/or of less than 20 μm.    -   The thickness of the transition layer is less than 10 μm,        preferably less than 8 μm and preferably less than 6 μm.    -   The transition layer is composed of a silica layer continuously        adherent to the substrate.    -   The thickness of the coating is less than 300 μm, preferably        less than 250 μm.    -   The thickness of the coating, which is preferably substantially        constant, is greater than 10 μm, preferably greater than 30 μm.    -   The coating material exhibits a lower thermal conductivity        and/or a lower emissivity and/or a greater effusivity than that        of the substrate.    -   The ratio of the thermal conductivity of the material of the        substrate to the thermal conductivity of the coating material is        greater than 10 and/or less than 150.    -   The coating material exhibits a lower thermal conductivity than        10 W.m.° C.; preferably, it is a refractory material.    -   The silicon compound is a nonoxide compound.    -   The coating material comprises more than 50%, preferably more        than 80%, preferably more than 90%, more preferably        substantially 100%, of said at least one ceramic oxide, as        percentage by weight.    -   The coating material is chosen from the group formed by        zirconia, which is optionally doped, in particular with magnesia        or with yttrium oxide; cordierite; alumina; mullite;        alumina/magnesia spinel; aluminum titanate; yttrium oxide;        magnesium oxide; hafnium oxide and the mixtures of these        materials. Advantageously, the coated part then exhibits good        properties of resistance to adhesion and to scratches,        withstands corrosion well and exhibits a low exudation with        temperature.    -   The coating material comprises more than 80% of zirconia. The        coating material can in particular be zirconia or a doped        zirconia. Preferably, the coating material comprises less than        5% of, less than 1% of, less than 0.1% of, and even no, silicon        metal alloy compound and in particular silica.    -   The coating is covered, at least partially, preferably        completely, with a layer made of a compliant material according        to the eleventh aspect of the invention and/or with a first        shrink ring, in particular as described according to the first        aspect of the invention. Advantageously, the coating acts        efficiently as a barrier against diffusion, improving the        resistance to thermal shocks.    -   Preferably, the coating is covered, at least partially,        preferably completely, with a layer made of a compliant material        according to the eleventh aspect of the invention, itself        covered with a first shrink ring, in particular as described        according to the first aspect of the invention.    -   The substrate comprises a ceramic material.    -   The amount by weight of silicon carbide in the substrate is        greater than 80%, preferably greater than 90%. Preferably, said        material of the substrate is composed of silicon carbide.    -   The substrate exhibits a total porosity of less than 5%, of less        than 2%, indeed even of less than 1%.    -   The coated part is a part intended to act as firing support, in        particular for the firing of ceramic parts. In particular, the        coating can be applied to a flat surface of a firing support.    -   The coated part is manufactured according to a process in        accordance with any one of the third, fourth and fifth aspects        of the invention.    -   The substrate and said at least one ceramic oxide are chosen so        as to manufacture a “first part” of a device according to one or        more other aspects of the invention. A coated part according to        the sixth aspect of the invention can thus also comprise one or        more of the characteristics, possibly optional, of a “first        part” of a device according to one or more other aspects of the        invention.

The invention also relates to a coated part according to the inventionin the form of a firing support or of a shrink fitted part or of ashrink ring.

As explained above, this is because the coating can be used inparticular to coat a first part in order to prevent excessively rapidtransfers of heat with a first shrink ring during the fitting of thelatter.

The inventors have also discovered that, when a first shrink ring isfitted to a coated part according to the invention, the coating veryefficiently limits chemical reactions between the materials of the firstpart and of the first shrink ring. The coating thus acts as a barrierwhich limits the diffusion of material between the first part and thefirst shrink ring. The result of this is an improved resistance inthermal cycling.

In particular in order to limit these heat transfers and/or thesechemical reactions, but without being limited thereto, the followingoptional characteristics are preferred:

-   -   The first shrink ring is itself shrink fitted by means of a        second shrink ring.    -   The coating at least partially covers a cylindrical surface, in        particular with a circular transverse cross section, of the        first part and/or of the first shrink ring and/or, if        appropriate, of the second shrink ring.    -   The coating encircles said cylindrical surface.    -   The coating defines at least a portion, preferably all, of the        region of contact between said first shrink ring and said first        part and/or, if appropriate, between said second shrink ring and        said first shrink ring.    -   The coating is positioned at least over at least 50%, preferably        at least 80%, more preferably substantially 100%, of said region        of contact.    -   Preferably, the coating extends from the edge of the first part        or of the first shrink ring by which the first part penetrates        into the first shrink ring during the fitting.    -   A second part is attached to the first shrink ring.

The first part and/or the second part and/or the first shrink ringand/or the second shrink ring may or may not be a first part and/or asecond part and/or a first shrink ring and/or a second shrink ring of adevice according to one or more other aspects of the invention,respectively. A device according to the sixth aspect of the inventioncan thus also comprise one or more of the characteristics, possiblyoptional, of a device according to one or more other aspects of theinvention.

Generally, the invention relates to an assembly comprising a coated partas described above and a shrink ring fitted to said coated part, saidcoated part comprising a substrate made of a material based on a siliconcompound and a coating made of a coating material comprising at leastone ceramic oxide, a transition layer comprising silica extendingbetween the substrate and the coating, said coating defining at least aportion of the region of contact between said shrink ring and saidcoated part.

The invention also relates to the application of a coating as definedabove between a first part and a first shrink ring fitted to the firstpart as “antidiffusion barrier”, that is to say in order to limit themigration of material between said first part and said first shrinkring.

SEVENTH ASPECT OF THE INVENTION

Thus, according to a seventh aspect, the invention also relates to aprocess for shrink fitting a first part, in particular made of a ceramicmaterial, by means of a first shrink ring, in which process, beforefitting the first shrink ring to the first part, a coating made of amaterial exhibiting a lower thermal conductivity and/or a greatereffusivity than that of the first part is applied to the first partand/or to the first shrink ring.

The coating is preferably applied to the first part when the firstshrink ring is heated in order to allow the fitting.

The coating is preferably applied according to a process for coating asubstrate in accordance with the invention.

Preferably, this process is adjusted in order for the first part to be acoated part in accordance with the invention.

EIGHTH ASPECT OF THE INVENTION

During their tests, the inventors also found that the joining of aceramic tube to a metal tube by means of one or more shrink rings, asdescribed according to the first aspect of the invention, can result inbreakages in the ceramic tube.

There thus exists a need for a solution which makes it possible to limitdamage to a part made of ceramic material when it is shrink fitted bymeans of a first shrink ring.

An objective of the invention is to meet this need.

According to an eighth aspect of the invention, this objective isachieved by means of a device comprising a first part made of ceramicmaterial shrink fitted by means of a first shrink ring, in which devicethe edges of the axial ends of the cylindrical portion of said firstpart and of the first shrink ring by which the first shrink ring hasbeen fitted to the first part belong to one and the same transverseplane.

In other words, the first shrink ring extends up to the edge of thecylindrical portion of said first part to which it has been fitted butwithout projecting axially therefrom. For the sake of clarity, it issaid that the first shrink ring is fitted “flush” to the first part.

In the case of an end-to-end joining of two tubes or of two bars, thistype of fitting is novel, the search to firmly attach the tubes or barsto one another naturally prompting at least the first shrink ring to befitted in overlapping fashion to these two tubes or bars. However, theinventors have found that a “flush” fitting very significantly limitsthe damage to the first part during the fitting of the first shrinkring.

It is said that two edges “belong to one and the same transverse plane”when their axial offsetting is less than 1 mm. Preferably, according tothe invention, this offsetting is less than 0.8 mm, preferably less than0.5 mm, more preferably less than 0.3 mm, still preferably less than 0.1mm. In the case where one or both of said edges is/are not included in atransverse plane, it is considered that the two edges “belong to one andthe same transverse plane” when, whatever the point of an edge underconsideration, the distance from the other edge is less than 1 mm,preferably less than 0.8 mm, preferably less than 0.5 mm, morepreferably less than 0.3 mm, still preferably less than 0.1 mm.

The term “transverse plane” describes a plane perpendicular to the axisof the first shrink ring.

A device according to the eighth aspect of the invention can optionallyalso comprise one or more of the following characteristics:

-   -   A second part is attached, preferably welded or brazed, to the        first shrink ring.    -   The first shrink ring is itself, preferably, shrink fitted by        means of a second shrink ring.    -   The second shrink ring is fitted flush to the first shrink ring.        In other words, the edges of the axial ends of the first shrink        ring, of the second shrink ring and of the first part belong to        one and the same transverse plane.    -   A second part is attached to the first shrink ring, preferably        so as to define, in interaction with the first shrink ring, a        chamber, preferably a leaktight chamber, in which the second        shrink ring is housed.

The first part and/or the second part and/or the first shrink ringand/or the second shrink ring may or may not be a first part and/or asecond part and/or a first shrink ring and/or a second shrink ring of adevice according to one or more other aspects of the invention,respectively. A device according to the eighth aspect of the inventionmay thus also comprise one or more of the characteristics, possiblyoptional, of a device according to one or more other aspects of theinvention.

NINTH ASPECT OF THE INVENTION

In one embodiment of a device according to the first aspect of theinvention, the second part is attached to the first shrink ring bywelding or brazing. This is because this type of connection isadvantageously mechanically resistant and lasting. However, theinventors have found that, when the first part is made of ceramicmaterial, it may break during the attaching of the second part.

There thus exists a need for a solution which makes it possible to limitthis risk of breakage.

An objective of the invention is to meet this need.

According to a ninth aspect of the invention, this objective is achievedby means of a device comprising a first part made of ceramic materialshrink fitted by means of a first shrink ring and a second part attachedto said first shrink ring, for example by a heating process, the regionof junction between the first shrink ring and the second part being atany point separated by at least 1 mm from said first part. For example,in the embodiment of FIG. 3, this separation of at least 0.8 mm, indeedeven of at least 1 mm, corresponds to the radial separation δ betweenthe external surface of the ceramic tube and the weld bead which makesit possible to bond the metal tube to the first shrink ring.

As will be seen in more detail in the continuation of the description,the separation of the region of junction limits the interactions withthe first part during the attaching of the second part to the firstshrink ring. In particular, when the second part is welded or brazed tothe first shrink ring, this separation reduces the heat gradients withinthe first part.

A device according to the ninth aspect of the invention may optionallyalso comprise one or more of the following characteristics:

-   -   The region of junction extends at right angles with the first        part. In particular, when the first part is a tube, said        separation is a radial separation, measured along a straight        line included in a transverse plane and passing through the axis        of the tube.    -   Said separation, in particular when it is radial, is greater        than 0.8 mm, greater than 1 mm, preferably greater than 3 mm,        greater than 5 mm, more preferably greater than 10 mm, or even        greater than 15 mm.    -   The second part is welded or brazed to the first shrink ring.    -   The first shrink ring exhibits a flange, continuous or        noncontinuous, preferably continuous, to the edge of which the        second part is attached.    -   The flange is continuous and encircles said first shrink ring.    -   The flange is provided at an axial end of said first shrink        ring.    -   The flange is positioned at the axial end of the first shrink        ring by which the first shrink ring has been fitted (that is to        say, introduced axially) to the first part.    -   The first shrink ring is itself shrink fitted by means of a        second shrink ring.

In a particularly advantageous embodiment, the first shrink ring is notfitted flush to the first part but projects axially therefrom and thesecond part is attached to the projecting portion of the first shrinkring, preferably at the end of the first shrink ring opposite the end bywhich it has been slipped onto the first part.

Advantageously, this embodiment does not require the provision of aflange, which simplifies the manufacture of the first shrink ring. Inaddition, it allows a minimal space requirement.

Preferably, the optional second shrink ring is, however, fitted flush tothe first part, that is to say that it extends up to the edge of thecylindrical part of said first part to which it has been fitted.

The first part and/or the second part and/or the first shrink ringand/or the second shrink ring may or may not be a first part and/or asecond part and/or a first shrink ring and/or a second shrink ring,respectively, of a device according to one or more other aspects of theinvention. A device according to the ninth aspect of the invention maythus also comprise one or more of the characteristics, possiblyoptional, of a device according to one or more other aspects of theinvention.

TENTH ASPECT OF THE INVENTION

The inventors have also sought to increase the lifetime of a deviceaccording to the first aspect of the invention.

According to a tenth aspect of the invention, they have thus discovereda device comprising a first part made of a ceramic material shrinkfitted by means of a first shrink ring, in which at least a portion ofthe first shrink ring, preferably at least the portion of the firstshrink ring in contact with the first part, indeed even all of the firstshrink ring, is protected by covering by means of a shield.

Preferably, the first shrink ring is itself shrink fitted by means of asecond shrink ring and at least a portion of the second shrink ring,preferably at least the portion of the second shrink ring in contactwith the first shrink ring, indeed even all of the second shrink ring,is protected by covering by means of a shield.

As will be seen in more detail in the continuation of the description,the installation of a protective shield at least partially covering ashrink ring makes it possible to limit the damage thereto, in particularby mechanical attacks, and thus to increase the lifetime of the device.

A device according to the tenth aspect of the invention can optionallyalso comprise one or more of the following characteristics:

-   -   The shield encircles the first shrink ring and/or the second        shrink ring.    -   The shield is not in contact with the portion of the first        shrink ring in contact with the first part.    -   The shield is not in contact with the second shrink ring.    -   The shield, preferably in interaction with the first shrink        ring, delimits a chamber, preferably an annular chamber.    -   The second shrink ring is housed in said chamber.    -   The chamber is airtight.    -   The chamber can exhibit any cross section. It can extend over        all or a portion of the periphery of the portion of the first        part to which the first shrink ring is attached.    -   The chamber contains a cooling or refrigerating or heat-exchange        or thermally insulating or chemically insulating fluid and/or        measurement means, in particular means for measuring the        pressure and/or the temperature, and/or detection means and/or        means for analyzing the environment.    -   A second part is attached, preferably welded, to the first        shrink ring, preferably via said shield.    -   The shield forms a portion of the second part and is preferably        integral with the second part.    -   The shield, indeed even the second part, is attached, preferably        welded, to a flange of the first shrink ring, preferably to the        free edge of said flange, said flange preferably being integral        with the first shrink ring.    -   The flange is positioned at the end of the first shrink ring by        which the first shrink ring has been fitted to the first part.        If appropriate, the flange is preferably positioned on the side        of the first shrink ring opposite, with respect to the second        shrink ring, the axial end of the cylindrical portion of the        first part by which the first shrink ring has been fitted to the        first part.    -   The second shrink ring is positioned axially between the flange        and the edge of the axial end of the first part by which the        first shrink ring has been fitted to the first part.    -   The amount by weight of silicon carbide in the material of the        first part is greater than 80%.

The first part and/or the second part and/or the first shrink ringand/or the second shrink ring may or may not be a first part and/or asecond part and/or a first shrink ring and/or a second shrink ring,respectively, of a device according to one or more other aspects of theinvention. A device according to the tenth aspect of the invention maythus also comprise one or more of the characteristics, possiblyoptional, of a device according to one or more other aspects of theinvention.

ELEVENTH ASPECT OF THE INVENTION

During cleaning operations, the pressure inside the tubes of an ethyleneproduction unit can be considerably increased and can typically reachseveral bar, During the thermal cycling, the leaktightness of theconnection by the shrink rings may also be challenged by the pressure ofthe reactive mixture, typically greater than 2.5 bar, at temperatures ofgreater than 900° C., indeed even of greater than 1000° C.

The inventors have thus looked for additional solutions in order toimprove the leaktightness of the connection between two parts subjectedto such stresses and in particular between two parts exhibiting verydifferent thermal expansion coefficients.

According to an eleventh aspect of the invention, they have thusdiscovered a device comprising a first member joined to a second memberpreferably exhibiting a greater thermal expansion coefficient than thefirst member, the joining being carried out via a compliant materialcomposed of an alloy comprising at least two materials from silver, goldand palladium. The inventors have found that said compliant materialwithstands very well an environment subjected to high variations intemperature while providing good cohesion of the joined members andleaktightness of the connection.

A device according to the eleventh aspect of the invention canoptionally also comprise one or more of the following characteristics:

-   -   The alloy comprises, indeed even is composed of:        -   silver and/or gold and        -   palladium.    -   The alloy comprises more than 0.5% of palladium, indeed even        more than 3% of palladium, as percentage by weight.    -   The alloy comprises less than 50% of palladium, as percentage by        weight.    -   The alloy comprises less than 30% of palladium, as percentage by        weight.    -   The alloy comprises less than 5% of glass frit.    -   The thickness of compliant material between the two members is        less than 1 mm and/or greater than 5 μm, preferably greater than        10 μm.    -   The second member is a shrink ring of the first member.    -   The second member is itself shrink fitted by means of a third        member.    -   The material of the third member exhibits a content by weight of        niobium and/or of Dilver P1 of greater than 80%.

The joining of the third member to the second member is carried out viaa layer made of said compliant material.

-   -   The material of the second member exhibits a greater thermal        expansion coefficient than the material of the first member and,        if appropriate, the third member exhibits a lower thermal        expansion coefficient than the thermal expansion coefficient of        the second member, between 20° C. and 1000° C.    -   The amount by weight of silicon carbide in the material of the        first member is greater than 80%.    -   A fourth member is attached to the second member.    -   The first member is coated with a coating made of a refractory        material.    -   The refractory material is preferably chosen from zirconia, a        doped zirconia, cordierite, alumina, mullite, aluminum titanate,        yttrium oxide, magnesium oxide, hafnium oxide or a mixture of        these materials.    -   The first member is coated with a coating made of a material        exhibiting a lower thermal conductivity than that of the        material of the first member.    -   The coating is in accordance with that described according to        the second aspect of the invention.

The invention also relates to a process for joining a first member and asecond member via a compliant material interposed between a firstsurface of the first member and a second surface of the second member,in which process:

-   -   a) at least one of the first and second surfaces is coated by        means of a precursor of a compliant material as defined above;    -   b) the first surface is pressed against the second surface with        a pressure of greater than 5 MPa, and    -   c) preferably simultaneously with stage b), said first and        second surfaces in contact are heated to a temperature of        greater than 150° C.

Preferably, in stage a), both the first surface and the second surfaceare coated with the compliant material precursor. Preferably again, instage b), the pressure is exerted by shrink fitting the first member bymeans of the second member.

The compliant material precursor can in particular be a suspension ofpowders formed of silver and/or of gold and of palladium.

The first member and the second member can in particular be a first partand a first shrink ring, respectively, or a first shrink ring and asecond shrink ring, respectively, of a device according to one or moreother aspects of the invention. In the presence of a first part and oftwo shrink rings, the compliant material precursor can in particular bepositioned so that the compliant material extends between the first partand the first shrink ring and between the first shrink ring and thesecond shrink ring. The third member can also be a second shrink ring. Adevice according to the eleventh aspect of the invention can alsocomprise one or more of the characteristics, possibly optional, of adevice according to one or more other aspects of the invention.

The invention also relates to an item of equipment chosen from afurnace, a boiler, a superheater, a steam generator, a chemical reactorand a heat exchanger, intended in particular for the production ofethylene, said item of equipment comprising a device according to anyone of the aspects of the invention. In particular, said item ofequipment can comprise at least one tube made of a material based on asilicon compound, in particular made of silicon carbide, attached by atleast one of its ends to a metal tube,

-   -   the tube made of a material based on a silicon compound and the        metal tube respectively constituting a first part and a second        part of a device according to any one of the aspects of the        invention and/or    -   the tube made of a material based on a silicon compound        constituting a coated part according to the sixth aspect of the        invention and/or    -   the tube made of a material based on a silicon compound or a        first shrink ring fitted to the tube made of a material based on        a silicon compound constituting a first member of a device        according to the eleventh aspect of the invention and/or    -   the tube made of a material based on a silicon compound, the        first shrink ring fitted to the tube made of a material based on        a silicon compound, the second shrink ring fitted to the first        shrink ring and the metal tube constituting a first member, a        second member, a third member and a fourth member, respectively,        of a device according to the eleventh aspect of the invention.

TWELFTH ASPECT OF THE INVENTION

The inventors have also sought to improve the quality of the connectionof a ceramic tube with a metal tube of a connecting device according tothe invention, in particular in an environment such as that of anethylene production furnace. They have thus discovered particularlyadvantageous adjustments.

If:

-   -   ED denotes the external diameter of the assembly formed by the        ceramic tube, optionally the coating made of a material        exhibiting a lower thermal conductivity than that of the        material of the ceramic tube (according to the second aspect of        the invention) and optionally the layer made of compliant        material between the ceramic tube and the first shrink ring        (according to the eleventh aspect of the invention),    -   ED₄₂ denotes the external diameter of the first shrink ring,    -   ID₄₂ denotes the internal diameter of the first shrink ring,    -   ID₄₂ denotes the internal diameter of the second shrink ring,    -   Δ₁ denotes the ratio 100*(ED−ID₄₂)/ED, and    -   Δ₂ denotes the ratio 100*(ED₄₂−ID₄₄)/ED₄₂,        all the measurements being carried out at 20° C., Δ₁ and/or Δ₂,        preferably Δ₁ and Δ₂, are greater than or equal to 0.00 and/or        less than 0.25, indeed even less than or equal to 0.20.

Preferably, Δ₁ is greater than or equal to 0.05.

BRIEF DESCRIPTION OF THE FIGURES

Other characteristics and advantages of the invention will become moreapparent on reading the detailed description which will follow and fromthe appended drawing, in which:

FIG. 1 diagrammatically represents an ethylene production unit;

FIG. 2 represents, in transverse cross section, a shrink ring and ashrink fitted part, before and after joining;

FIG. 3 represents, in perspective and cut by a median longitudinalplane, a device according to a preferred embodiment of the invention;

FIGS. 4 to 6 illustrate the various stages of a process used to jointogether the device represented in FIG. 3; and

FIG. 7 represents an alternative form of a device according to theinvention.

In the various figures, identical references are used to denoteidentical or analogous members.

DETAILED DESCRIPTION

FIG. 1 was described in the preamble of the description.

Shrink fitting is a technique for the axial fitting of an external partto an internal part which makes use of the difference in dilatometricbehavior between these two parts. The external part is conventionallyreferred to as “shrink ring”. The internal part is referred to as“shrink fitted”.

More specifically, the shrink ring is made of a material exhibiting agreater thermal expansion coefficient than that of the internal part. Asrepresented in FIG. 2, the shrink ring F exhibits an orifice O,delimited by an internal surface, the dimensions of which make itdifficult, indeed even impossible, to fit it to the internal part P byhand or even, conventionally, using a press. When the shrink ring F iscylindrical with a circular transverse cross section, its internaldiameter ID is thus less than the external diameter OD of the internalpart P in the range of temperatures anticipated for the use of thejoined components. In this range, the fitting of the shrink ring is thusimpossible. For this, the shrink ring F has to be brought to atemperature greater than that of the internal part P. This difference intemperatures can be obtained by heating the shrink ring F and/or bycooling the internal part P, for example with liquid nitrogen or withdry ice. The difference in temperatures has to be determined in orderfor the dimensions of the orifice O of the shrink ring F to then allowit to be introduced over the internal part.

Once the shrink ring F has been introduced over the internal part P, thecombination is brought to the same temperature, for example to thestarting ambient temperature, which results in the shrink ring Fcompressing the internal part and provides the cohesion between theshrink ring F and the internal part P. However, this cohesion may bechallenged if the temperature of the surroundings of the joinedcomponents increases. This is because the shrink ring will then expandmore rapidly than the internal part. An increase in the temperature maythus result in a loss of leaktightness, indeed even in an unintentionalseparation of the parts.

In particular, under the conditions of variation in temperature of afurnace of an ethylene production unit, the inventors have found thatjoining a metal tube by shrink fitting to a ceramic tube isunsatisfactory. This is because the thermal expansion coefficients ofthe materials of these tubes are too different for the shrink fitting tobe able to provide a reliable connection over a range of temperatureswith an amplitude of more than 1000° C.

U.S. Pat. No. 4,624,484 describes a connecting device which makes itpossible to join a ceramic part by means of a shrink ring. Beforefitting the shrink ring, a layer made of a nonferrous metal is appliedto the internal surface of the shrink ring intended to be brought intocontact with the ceramic part. After fitting, this layer deforms so asto compensate for the differences in expansion between the ceramic tubeand the shrink ring. Such a layer, or generally a coating, are notshrink rings.

The inventors then imagined providing the connection between the ceramictube and the metal tube by means of several intermediate shrink ringsexhibiting gradually increasing thermal expansion coefficients: a firstshrink ring exhibiting a slightly greater thermal expansion coefficientthan that of the ceramic tube was fitted to this ceramic tube, a secondshrink ring exhibiting a slightly greater thermal expansion coefficientthan that of the first shrink ring was fitted to this first shrink ring,this type of fitting being reproduced until a shrink ring is obtainedexhibiting a thermal expansion coefficient similar to that of the metaltube. The metal tube was subsequently attached to this final shrinkring. However, tests have shown that, under the conditions of variationin temperature of a furnace of an ethylene production unit, thistechnical solution results in the application of high stresses to theceramic tube and can result in the breaking thereof.

The inventors then devised and produced a connecting device of anentirely different design.

In one embodiment, this connecting device comprises a first shrink ringfitted to the ceramic tube and a second shrink ring fitted to the firstshrink ring, the second shrink ring exhibiting a lower thermal expansioncoefficient than that of the first shrink ring. In the event of anincrease in the temperature, the second shrink ring thus opposes theexpansion of the first shrink ring and thus prevents a loss inleaktightness or a separation from the first shrink ring. In addition,this type of joining has proved to be particularly reliable under theconditions of an ethylene production furnace.

FIG. 3 represents an example of joining by means of a connecting deviceaccording to a preferred nonlimiting embodiment of the invention.

For the sake of clarity, the tubes represented in FIG. 3 are regarded aspositioned along a vertical axis V. Without implied limitation, thewords “top”, “bottom”, “upper” and “lower” are used to describe membersor member portions as a function of their position with respect to thisaxis.

FIG. 3 represents the lower portion 20 of a ceramic tube 22 with an axisX connected coaxially to the upper portion 24 of a metal tube 26 bymeans of a connecting device 30.

The ceramic tube 22 and the metal tube 26 exhibit, except at theconnecting device, identical annular transverse cross sections. Theexternal diameter of the tubes 22 and 26 is preferably greater than 50mm and/or less than 100 mm. The internal diameter of the tubes 22 and 26is preferably greater than 40 mm and/or less than 50 mm.

The ceramic tube 22 is made of a nonoxide material, preferably ofsilicon carbide, and preferably exhibits a porosity of less than 1%,this porosity being closed.

The lower portion 20 of the ceramic tube 22 is coated with a thermallyinsulating coating 32, for example made of zirconia. The coating 32extends along an annular surface intended to act as support for a firstshrink ring which will be described below. It exhibits a thickness ofapproximately 30 μm.

Photographs show, at the interface between the silicon carbide of theceramic tube 22 and the zirconia of the coating 32, the presence of atransition layer with a substantially constant thickness ofapproximately 5 μm. The transition layer is composed of a silica layercontinuously adherent to the ceramic tube 32.

Except in its upper portion 24, the metal tube 26 is a tube similar tothe tubes used in the prior art upstream or downstream of the furnacesused for the production of ethylene.

The upper portion 24 of the metal tube 26 comprises a wall 34, thethickness of which gradually broadens until a maximum is reached in atransverse plane P₁. In this plane, the metal tube 26 exhibits a flatannular surface 36, on which will rest a lower edge 38 of the ceramictube 22 and/or the lower edge of the first shrink ring described above,in the nonlimiting embodiment of FIG. 3. A cylindrical peripheral skirt40 with a circular cross section with an axis X extends upward from theperiphery of the annular surface 36.

The connecting device 30 comprises a first shrink ring 42 and a secondshrink ring 44.

The first shrink ring 42 is composed of a material exhibiting a thermalexpansion coefficient of less than 25.10⁶ K⁻¹ and greater than 10.10⁻⁶K⁻¹ between 20 and 1000° C. and is preferably composed of the samematerial as the metal tube 26, for example of HP40 steel or of an FeNiCralloy. The use of the same material to manufacture the metal tube 26 andthe first shrink ring 42 advantageously facilitates the welding thereofto one another.

The first shrink ring 42 exhibits a total length L₄₂ of approximately 60mm and a substantially constant wall thickness e₄₂ preferably between 1and 5 mm, preferably of approximately 2 mm. The actions of the secondshrink ring on the first shrink ring and of the first shrink ring on theceramic tube 22 are then optimal in the event of an increase in thetemperature from 20° C. to 900° C.

In one embodiment, the first shrink ring 42 exhibits a substantiallyconstant wall thickness e₄₂ of greater than 3 mm, indeed even greaterthan 3.5 mm, indeed even substantially equal to 4 mm. This is becausethe inventors have discovered that the thickness of the first shrinkring modifies the quality of the leaktightness of the connection atambient temperature, the best results having been obtained with athickness e₄₂ of approximately 4 mm.

The first shrink ring 42 exhibits the shape of a cylindrical sleeve 46of circular cross section and with a length L₄₆ of approximately 50 mm,which is extended upward by a flange 48.

Over its entire length, the cylindrical sleeve 46 is in contact with thecoating 32 of the ceramic tube 22 via a layer 50 of a compliant materialwhich improves the leaktightness between the ceramic tube and the firstshrink ring. The layer 50 exhibits a substantially constant thickness ofapproximately 30 μm.

The compliant material comprises, on the one hand, silver and/or goldand, on the other hand, palladium. The inventors have discovered thatsuch a material is particularly reliable. In particular, this materialexhibits a very good refractoriness and a very high melting point whichrender it compatible with the stresses encountered in a furnace of anethylene production unit.

For example, the compliant material comprises approximately 30% ofpalladium, the remainder being silver.

The cylindrical sleeve 46 defines a lower edge 52 of the first shrinkring 42. The lower edge 52 of the cylindrical sleeve 46 is coplanar withthe lower edge 38 of the ceramic tube 22 and may be in contact with theannular surface 36. This is because the inventors have discovered thatthis configuration limits the risks of breakage of the ceramic tube 22.

The flange 48 results from a gradual opening out until the wall of thefirst shrink ring extends substantially perpendicular to the axis X.This gradual opening out limits the risks of breakage of the ceramictube during the fitting of the first shrink ring 42 and facilitates theintroduction of the ceramic tube 22 into the first shrink ring 22. Theflange 48 defines a circular “upper edge” 56 of the first shrink ring42.

The upper edge 58 of the peripheral skirt 40 is attached to the upperedge 56 of the flange 48 by means of a weld bead 60. The radialseparation 5 between the external surface 61 of the ceramic tube 22 andthe weld bead 60 is, for example, approximately 5 mm.

The annular surface 36, the peripheral skirt 40 and the first shrinkring 42 thus define a chamber 62. The weld bead 60 is preferablycontinuous, so that the chamber 62 is substantially leaktight.Optionally, the chamber 62 is also isolated from the internal volume ofthe ceramic tube 22 and the metal tube 26.

The internal volume of the chamber 62 is a few tens of cm³, preferablyof more than 20 cm³, of more than 50 cm³, and/or preferably of less than100 cm³, of less than 80 cm³, preferably of approximately 70 cm³.Advantageously, the optional accumulation of coke is thus limited.

The second shrink ring 44 is composed of a material exhibiting a lowerthermal expansion coefficient than that of the first shrink ring. Thedifference in the thermal expansion coefficients between the firstshrink ring and the second shrink ring is determined as a function ofthe application. When the connecting device is intended to be subjectedto variations in the temperatures such as those encountered inside thechamber of an ethylene production unit furnace or immediately upstreamor downstream of such a chamber, the second shrink ring can inparticular be made of an alloy comprising more than 70% of niobiumand/or of Dilver P1. Such a niobium and/or Dilver P1 alloyadvantageously exhibits a low thermal expansion coefficient, a lowYoung's modulus (modulus of elasticity) and a good resistance to heatand to corrosion at high temperature, it being possible for the latterto be further improved by the presence of a coating. For example, thematerial of the second shrink ring comprises 89% of niobium, 10% ofhafnium and 1% of titanium. The resistance to corrosion can be furtherimproved by coating the second shrink ring with a protective coating, or“coating” of the “coating 32” type. The second shrink ring 44 exhibitsthe shape of a cylindrical sleeve of circular cross section and isfitted to the first shrink ring by shrink fitting via a layer 63 ofcompliant material, for example identical to that extending between theceramic tube and the first shrink ring.

The length L₄₄ of the second shrink ring 44 is substantially identicalto that of the cylindrical sleeve 46 of the first shrink ring 42. Thethickness e₄₄ is between 3 and 15 mm, preferably between 5 and 13 mm.The action of the second shrink ring on the first shrink ring is thenoptimal in the event of an increase in the temperature.

Furthermore, the second shrink ring 44 is housed inside the chamber 62.Advantageously, the second shrink ring is thus protected from theoutside world. The peripheral skirt 40 thus acts as protective shieldfor the second shrink ring 44 but also for the first shrink ring 42.

The lower edge 64 of the second shrink ring 44 is substantially coplanarwith the lower edge 38 of the ceramic tube 22 and the lower edge 52 ofthe first shrink ring 42. This is because this configuration has provedto be optimal in limiting the risk of breakage of the ceramic tubeduring the fitting of the second shrink ring.

The upper edge 65 of the second shrink ring 26 is beveled toward theinside and toward the outside.

A connecting device such as that represented in FIG. 3 can be producedin the following way:

The lower portion of the ceramic tube 22 is first covered with theinsulating coating 32. However, the adhesion of a coating made ofceramic oxide to a tube made of a nonoxide material is difficult, inparticular when the nonoxide material is not or only slightly porous.

In particular, conventional techniques for the direct spraying of veryfine particles of said ceramic oxide by means of a plasma torch haveproved to be ineffective in the case of spraying onto a first part madeof nonoxide ceramic.

However, the inventors have discovered several novel processes forcoating a substrate based on a silicon compound by means of a coatingcomprising at least one ceramic oxide. In particular, they havediscovered a process for coating a substrate based on a silicon compoundby means of a coating comprising at least one ceramic oxide, comprisingthe following successive stages:

-   -   a1) application to said substrate, preferably at ambient        temperature, of particles comprising, preferably composed of,        said at least one ceramic oxide;    -   b1) heat treatment of said substrate under conditions suitable        for causing said at least one ceramic oxide to react with        silicon of the substrate, so as to form silica.

Preferably, the particles of said at least one ceramic oxide are applieddirectly to the substrate, without an intermediate layer. In particular,no intermediate layer of silicon or of silica is positioned on thesubstrate before stage a1). The thickness of the transition layer formedduring stage b1) is then advantageously very low, typically of less than10 μm, indeed even less than 8 μm or even of less than 6 μm.

In a preferred embodiment, a process according to the invention does notcomprise a stage which makes it possible to form silica before stagea1).

In stage a1), all conventional techniques can be used to apply theparticles. In particular, the particles of said ceramic oxide can beapplied by spraying, painting or dipping. To this end, a slip isprepared and is applied to the substrate. The slip conventionallycomprises a solvent, preferably water, an organic dispersant and saidparticles.

Preferably, the slip does not comprise silica and preferably does notcomprise a silicon compound. After firing, the silica will thus bepresent only in the transition layer, a coating devoid of silica beingexposed to the outside world. The absence of silica may modify thepermeability of the coating but, in the embodiment represented, thismodification has no practical effect, the coating being itself coveredwith a layer made of compliant material and/or with a first shrink ring.In addition, the manufacturing process is simplified thereby.

Application can be carried out at ambient temperature, for example inthe open air. The amount of particles is adjusted according to thecoating thickness desired.

Preferably again, the process does not comprise any stage between stagesa1) and b1).

In stage b1), the substrate is preferably subjected to firing at atemperature of between 1000° C. and 1400° C., preferably for a time ofgreater than 1 hour, for example under air.

Surprisingly, the heat treatment results in the formation, in thesubstrate, of a transition layer in which a portion of the siliconcompound has reacted to form a silica layer within the material of thesubstrate. Without being able to explain it theoretically, the inventorshave found that the silica layer considerably improves the quality ofthe adhesion of the ceramic oxide.

In a preferred embodiment, a process according to the inventioncomprises only stages a1) and b1). The silica of the transition layerthus results exclusively from the implementation of stage b1). Theprocess is then particularly easy to carry out.

The inventors have also developed alternative forms of this process.

In particular, they have discovered a process for coating a substratebased on a silicon compound by means of a coating comprising at leastone ceramic oxide, this process comprising the following successivestages:

-   -   a2) oxidation, at least at the surface of said substrate, of        said silicon compound, for example by a heat treatment under an        oxidizing atmosphere, so as to form silica,    -   b2) application to said substrate, preferably at ambient        temperature, of particles comprising, preferably composed of,        said at least one ceramic oxide;    -   c2) heat treatment of said substrate under conditions suitable        for causing said at least one ceramic oxide to react with said        silica.

In stage a2), the oxidative heat treatment can, for example, be carriedout by heating the surface of the substrate at a temperature of 1250° C.for 3 hours, for example under air, for example at atmospheric pressure.

In one embodiment, only a portion of the substrate, in particular onlyits surface or only the section of its surface intended to receive thecoating, is heated. For example, in the case of a ceramic tube, it isnot always necessary to introduce all the tube into the furnace.

Preferably, only a surface layer of the substrate is oxidized. Morepreferably, the substrate is oxidized only over a depth of less than 20μm.

In stage b2), the particles of said at least one ceramic oxide can beapplied as in stage a1).

In stage c2), the substrate is preferably subjected to firing at atemperature of between 1000° C. and 1400° C., preferably for a time ofgreater than 1 hour, for example under air.

The inventors have also invented a process for coating a substrate basedon a silicon compound by means of a coating comprising at least oneceramic oxide, this process comprising the following successive stages:

-   -   a3) deposition of a silicon layer at the surface of the        substrate, preferably by plasma spraying;    -   b3) application to said silicon layer, preferably at ambient        temperature, of particles comprising, preferably composed of,        said at least one ceramic oxide;    -   c3) optionally heat treatment of said substrate in order to        cause said at least one ceramic oxide to react with silicon of        the substrate, so as to form silica.

In stage a3), a conventional plasma torch can be employed.

In stage b3), the particles of said at least one ceramic oxide can beapplied as in stage a1).

Stage c3) can be carried out as in stage b1). However, in this process,advantageously no heat treatment is necessary if, in stage b3), theapplication of the particles is carried out by thermal spraying. Theprocess is advantageously simplified thereby.

It is possible to cause the coating 32 of zirconia to adhere to thesubstrate formed by the lower portion of the ceramic tube 22 byfollowing one of the three processes which have just been described.

Before installing the first shrink ring 42 on the first part, a layer ofcompliant material precursor is applied to the external surface of thelower portion 20 of the ceramic tube 22 and/or to the internal surface(intended to come into contact with the lower portion 20 of the firstshrink ring 42.

The compliant material precursor can be prepared in the form of asuspension in an organic solvent. This suspension is then applied, forexample by spraying, painting or dipping. The solvent is then removed,for example by drying.

The particle size distribution of the powder of the suspension and theamount and the nature of the solvent are adjusted according to thenature of the surfaces and the thickness of compliant material which aredesired.

The fitting of the first shrink ring 42 to the lower portion 20 of theceramic tube 22 is subsequently carried out, preferably by expanding thefirst shrink ring by heating and by then introducing it axially, by itsaxial end provided with the flange 48, over the ceramic tube 22.

Preferably, the first shrink ring is fitted “flush” to the lower portion20 of the ceramic tube 22. In other words, it is immobilized in aposition in which its lower edge 52 is aligned, in one and the sametransverse plane, with the lower edge 38 of the ceramic tube. Theinventors have found that this arrangement advantageously limits therisk of breakage of the ceramic tube.

As explained above, silicon carbide is highly conducting thermally andany contact between the first shrink ring 42 and the ceramic tube 22would result, in the absence of the coating 32, in the suddencontraction in the first shrink ring, resulting in it becoming lockedaxially before it could reach the definitive position desired.

The presence of the coating 32 limits the heat exchanges between thefirst shrink ring 42 and the ceramic tube 22, in particular in the eventof contact. This is because, during the introduction of the first shrinkring 42 over the ceramic tube 22, the insulating coating advantageouslyforms a thermal barrier which limits transfers of heat between the firstshrink ring 42 at high temperature and the thermally conducting ceramictube 22.

The term “clearance” J refers to the difference between the internaldiameter of the first shrink ring 42 and the external diameter of theceramic tube 22. At the time of the introduction of the first shrinkring 42 over the ceramic tube 22, the clearance J is positive and allowsthe fitting by shrink fitting of the first shrink ring to the ceramictube 22. By virtue of the presence of the coating, the clearance J canadvantageously be considerably reduced and/or the temperature of thefirst shrink ring decreased, without the risk of untimely axial locking.

The return to ambient temperature results in a greater contraction ofthe first shrink ring than that of the ceramic tube, which leads to acancellation of the clearance J and then to a pressure of the firstshrink ring on the ceramic tube 22 (FIG. 4). Advantageously, thiscompression also attributes to the effectiveness of the compliantmaterial. Before installing the second shrink ring 44 on the firstshrink ring 42, a layer of compliant material precursor is also appliedto the external surface of the first shrink ring 42 and/or to theinternal surface of the second shrink ring 44, at least over the areasof these surfaces intended to be in contact after shrink fitting.

The second shrink ring 44 can then be fitted to the first shrink ring 42by a conventional shrink fitting (FIG. 5). The shape beveled toward theinside of the upper edge 65 of the second shrink ring 44 facilitates theshrink fitting to the first shrink ring.

As for the fitting of the first shrink ring to the ceramic tube 22, acoating which forms a thermal barrier can also be applied to theexternal surface of the first shrink ring in order to limit theclearance necessary during the shrink fitting operation. Preferably, thesecond shrink ring is fitted “flush” to the first shrink ring. In otherwords, it is immobilized in a position in which its lower edge 64 isaligned, in one and the same transverse plane P₁, with the lower edge 52of the first shrink ring. The inventors have found that this positionadvantageously limits the risk of breakage of the ceramic tube.

The shape beveled towards the outside of the upper edge 65 of the secondshrink ring 44 also advantageously limits the stresses applied to theceramic tube 22.

The ceramic tube 22 is subsequently brought closer coaxially to themetal tube 26 until it abuts against the annular surface 36. As thefirst shrink ring 42 and the second shrink ring 44 are fitted flush withthe lower edge 38 of the ceramic tube 22, it is also possible for thesupport on the annular surface 36 to be achieved via the lower edge 52of the first shrink ring 42 and/or the lower edge 64 of the secondshrink ring.

In this abutment position, as represented in FIG. 6, the upper edge 58of the peripheral skirt 40 faces the upper edge 56 of the flange 48.These edges are then united by welding or brazing.

The production of the weld bead 60 conventionally involves temperaturesof approximately 1500° C. Advantageously, the region of junction (weldbead 60) between the upper edge 56 of the flange 48 and the upper edge58 of the peripheral skirt 40 is at a distance from the external surface61 of the ceramic tube 22. Advantageously, the risk of a thermal shockcapable of breaking the ceramic tube is limited thereby.

In this position, the first shrink ring 42 and the metal tube 26 delimitthe chamber 62. The second shrink ring is positioned in the chamber 62.Advantageously, it is thus protected from attacks, in particularphysical attacks.

Preferably, the weld bead 60 is substantially continuous, so that thechamber 62 is leaktight, thus also protecting the second shrink ringfrom chemical attacks.

The ceramic tube 22 is then reliably attached to the metal tube 26.

FIG. 7 represents a device according to the invention according to analternative form in which the first shrink ring 42 has the form of asleeve which projects beyond the axial end of the ceramic tube 22. Themetal tube is attached to the free end of the first shrink ring 42 bymeans of a weld bead 60, at a distance from the ceramic tube. The secondshrink ring 44 is fitted flush to the ceramic tube 22.

Alternative forms of the embodiment represented in FIG. 7 are possible.For example, the metal tube 26 can be welded end-to-end with the firstshrink ring 42, the first shrink ring and the metal tube being incontact edgewise. The internal diameter and/or the external diameter ofthe first shrink ring 42 may be constant or variable along the axis X.The metal tube 26 can also be attached to the external surface of thefirst shrink ring 42.

Tests have been carried out with a connecting device of the type of thatrepresented in FIG. 3. If:

-   -   ED: denotes the external diameter of the assembly formed by the        ceramic tube 22, the coating 32 and the layer 50 of compliant        material,    -   ED₄₂: denotes the external diameter of the first shrink ring 42,    -   ID₄₂: denotes the internal diameter of the first shrink ring 42,    -   ID₄₂: denotes the internal diameter of the second shrink ring        44,    -   Δ₁: denotes the ratio 100*(ED−ID₄₂)/ED, and

-   Δ₂: denotes the ratio 100*(ED₄₂−ID₄₄)/ED₄₂,    all the measurements being carried out at 20° C.,    the combinations of the following values of Δ₁ and Δ₂ resulted in    particularly reliable fittings:

TABLE 1 Example e₄₂ e₄₄ Δ₁ Δ₂ 1 2 10 0.20 0.20 2 2 12 0.10 0.20 3 2 100.20 0.10 4 2 10 0.10 0.10 5 3 10 0.10 0.15 6 4 10 0.10 0.10 7 4 10 0.050.00

Tests have also been carried out with the device of example 6, in orderto measure the advantage of a coating 32 at the surface of the ceramictube and of a layer 50 made of compliant material interposed between thecoating 32 and the first shrink ring 42.

The compliant material was made of an Ag/Pd alloy. The results obtainedare summarized in the following table 2:

TABLE 2 Results with thermal Example 6 cycling at 900° C. withoutcoating 32 or Breaking in the layer 50 second cycle with coating 32 ofNo breaking after 5 zirconia and layer 50 cycles and with coating 32 ofleaktightness alumina/zirconia and retained layer 50

These results show that the coating 32 and the layer 50 not only have aneffect on the leaktightness but also have an effect on the resistance tothermal cycling.

Of course, the invention is not limited to an embodiment described andrepresented.

In particular, a shrink fitting does not require that the shrink fittedpart exhibits, in a transverse cross section, a circular externaloutline. In one embodiment, the first part and/or the first shrink ringdo(es) not exhibit a circular outline. Advantageously, any rotation ofthe first shrink ring and/or of the second shrink ring, respectively,around the axis of the first shrink ring and/or of the second shrinkring, respectively, is then prevented. The first part and/or the firstshrink ring can in particular exhibit, in transverse cross section, anoblong outline or an outline exhibiting a plurality of points,preferably distributed at equal angles around the axis, for example astar shape.

1. A device comprising a first member joined to a second member, thejoining being carried out via a compliant material composed of an alloycomprising at least two materials selected from the group consisting ofsilver, gold, and palladium, the second member being a shrink ring ofthe first member.
 2. The device according to claim 1, wherein the alloycomprises palladium.
 3. The device according to claim 1, wherein thefirst member is coated with a coating made of a refractory material. 4.The device according to claim 3, wherein the refractory material isselected from the group consisting of zirconium, a doped zirconium,cordierite, alumina, mullite, aluminum titanate, yttrium oxide,magnesium oxide, hafnium oxide, and a mixture thereof.
 5. The deviceaccording to claim 1, wherein the first member is coated with a coatingmade of a material exhibiting a lower thermal conductivity than that ofa material of the first member.
 6. The device according to claim 1,wherein the second member exhibits a greater thermal expansioncoefficient than the first member.
 7. The device according to claim 1,wherein the alloy comprises more than 0.5% of palladium.
 8. The deviceaccording to claim 7, wherein the alloy comprises less than 50% ofpalladium by weight.
 9. The device according to claim 8, wherein thealloy comprises less than 30% of palladium by weight.
 10. The deviceaccording to claim 1, wherein the thickness of the compliant materialbetween the two members is less than 1 mm and greater than 5 μm.
 11. Thedevice according to claim 1, wherein the second member is shrink fittedwith a third member.
 12. The device according to claim 11, wherein thematerial of the third member exhibits a content by weight of niobium ofgreater than 80%.
 13. The device according to claim 12, wherein thejoining of the third member to the second member is carried out via alayer made of said compliant material.
 14. The device according to claim11, wherein: a material of the second member exhibits a greater thermalexpansion coefficient than a material of the first member at atemperature of from 20° C. to 1000° C.; and a third member exhibits alower thermal expansion coefficient than the thermal expansioncoefficient of the second member at a temperature of from 20° C. to1000° C.
 15. The device according to claim 1, wherein an amount byweight of silicon carbide in the material of the first member is greaterthan 80%.
 16. The device according to claim 1, wherein the alloyconsists of silver and/or of gold and of palladium.
 17. The deviceaccording to claim 1, wherein a fourth member is attached to the secondmember.
 18. An item of equipment selected from the group consisting of afurnace, a boiler, a superheater, a steam generator, a chemical reactor,and a heat exchanger, said item of equipment comprising at least onetube made of silicon carbide attached by at least one of its ends to ametal tube, the tube made of silicon carbide and the metal tuberespectively constituting a first member and a fourth member of a devicewherein the device is the device according to claim
 17. 19. A steamcracking unit comprising the item of equipment according to claim 18.20. A unit for the recovery of waste comprising the item of equipmentaccording to claim 18.